Polyethylene naphthalate (PEN) produced by the polymerization of 2,6-NDA and ethylene glycol are known to excel in several properties such as thermal stability, strength, measurement stability and gas permeability, etc. when compared with polyethylene terephthalate (PET). Based on these advantages, PEN is chosen as good materials for films, fibers and bottles, etc. In addition, PEN is highly resistant to diffusion of gases, it is useful for the manufacture of food containers and packaging materials. PEN can also be used as a material to replace high-priced tire cords. 2,6-naphthalene dicarboxylate (2,6-NDC) is a generally commercialized PEN monomer, as known in U.S. Pat. Nos. 5,254,719, 5,262,560, 6,211,398, and 6,013,831. At present, while 2,6-NDC is used as a major material of PEN, using 2,6-NDA is highly effective due to high yield and low production cost of polymers. Further, directly purifying crude-NDA prepared in the oxidation process, without converting it into 2,6-NDC, simplifies the process and lowers production costs. Moreover, in the polymerization process of 2,6-NDC, explosion-proof equipment is necessary because methanol is formed as a byproduct, whereas in the polymerization process of 2,6-NDA, water is formed, thus, such an additional equipment as above is unnecessary.
2,6-NDA is obtained by oxidizing 2,6-dimethyl naphthalene (hereinafter referred to as “2,6-DMN”) in the presence of a heavy metal catalyst. However, the thus-obtained crude 2,6-NDA contains a large amount of numerous impurities as by-products such as 6-formyl-2-naphthoic acid(FNA), 6-methyl-2-naphthoic acid(MNA), trimellitic acid(TMLA), bromo naphthalene dicarboxylic acid(Br-NDA) and 2-naphthoic acid(NA) as well as catalytic metals such as cobalt and manganese.
PEN obtained by the polymerization of ethylene glycol and 2,6-NDA containing various impurities as mentioned above shows poor quality due to its low thermal resistance and softening point, and coloration. Accordingly, high purity 2,6-NDA having a purity of about 99.9 wt % is required to obtain high quality PEN.
Several methods for refining impurities of NDA to obtain high purity NDA have been known. U.S. Pat. Nos. 5,256,817 and 6,255,525 disclose a method for refining impurities by dissolving NDA in general solvents and hydrogenating it. Furthermore, U.S. Pat. No. 5,256,817 discloses a method for refining NDA using an acetic acid or an aqueous solution of acetic acid as a solvent, dissolving it at a high temperature, and then hydrogenating it. However, an excessive amount of naphthoic acid and tetralin dicarboxylic acid is easily produced due to decarbonation, a by-product of hydrogenation reaction. And it also requires expensive metal catalysts for hydrogenation, thereby it being economically disadvantageous.
U.S. Pat. No. 6,255,525 discloses a process of filtering insoluble substances by using water as a solvent, and impurities thereof are removed or converted into a removable form by hydrogenation. However, the process has disadvantages in that the NDA thus prepared has a high chromaticity and the catalyst has a short life time.
Japanese Patent Publication No. 62-2307 discloses a method of dissolving NDA in solvents such as dimethylsulfoxide, dimethylformamide, etc., removing insoluble substances and then recrystallizing it. However, such method requires a quantity of solvents and activated carbons, and 2,6-NDA is rarely soluble in the solvents thereof. Further, the solvents may be hydrogenated with impurities, making hydrogenation impossible, and the formyl naphthoic acid is not eliminated completely.
Japanese patent Publication No. 5-32586 A discloses a method for refining 2,6-NDA by dissolving it using pyridine or pyridine derivatives as a solvent and then crystallizing it, but this method is also problematic because the solubility of 2,6-NDA in the solvents is sensitive to temperature and its yield is thus low.
Korean Patent Publication 2006-009437 discloses a method of refining 2,6-NDA, comprising the steps of: mixing crude 2,6-NDA and amine compounds, and solvents and non-solvents; heating and dissolving the compounds thus prepared; cooling and filtering the mixture to obtain an amine salt crystal of 2,6-NDA; and heating the amine salt crystal of 2,6-NDA to deaminate the salt. However, this method is too complicated and has a low yield per unit in each process. Moreover, the method is not economically advantageous as expensive amine compounds are used.
U.S. Pat. No. 5,563,294 discloses a method for producing 2,6-NDA by esterifying NDC, followed by removal of methanol and crystallization. But the method comprises multiple complicated processes, and as such is not economically efficient.
As another method for refining 2,6-NDA, there is a method of converting NDA into its metal salt and then dissolving and recrystallizing it. Japanese Patent Publication No. 52-20993 A discloses a method for refining 2,6-NDA by dissolving it in alkali aqueous solutions such as KOH or NaOH with pH adjusted to thereby prepare its alkali metal salt, then absorbing it with a solid absorbent and crystallizing it. Disproportionation of the thus-produced mono alkali salt with water generates a refined 2,6-NDA. However, this method requires a quantity of solid absorbents and solvents, and as all mono alkali salts are crystallized, salts generated by impurities such as NA or FNA, etc. are also crystallized, thus making it difficult to separate them. Further, alkali metal remains in the crystal of NDA, thereby deteriorating chromaticity when polymerizing, and causing disadvantages in terms of a crystal size and yield.
Therefore, a simple and easy process to obtain high purity 2,6-NDA having excellent color and purity as well as high yield in an economical manner is required.
The above-said refining methods use relatively complicated processes and expensive solvents, or additional reaction processes are necessary. Such methods produce impurities in the process due to by-products and additives, thereby leading to an additional process for treating such impurities, and making the methods economically disadvantageous.